Question

A student has obtained an image of a distant object on a screen to determine the focal length F1 of the given lens. His teacher, after checking the image, gave him another lens of focal length F2 and asked him to focus the same object on the same screen. The student found that to obtain a sharp image, he has to move the lens away from the screen. From this finding, we may conclude that both the lenses given to the student were :
(A) Concave and F1 < F2
(B) Convex and F1 < F2
(C) Convex and F1 > F2
(D) Concave and F1 > F2

Answer 1

Answer:

concave and F1<F2

Explanation:

because cobcave is F1and F2

Answer 2

(C) Convex and F1 > F2

Explanation:

Given that the image formed is real. So the lens is convex.

As the image distance is increasing, we infer that the object distance is decreasing.

1/ f  = 1 /v - 1/ u

f = uv/ u-v

For convex lens, object distance is -u and image distance is v.

So f = uv / u + v

Focal length is less for the second lens.

F2 is less than F1.

Option C is the answer.